Method for forming spatially-varying distributed Bragg reflectors in optical media

ABSTRACT

A method for forming a grating in a photosensitive medium such as a photosensitive optical fiber. The method comprises impinging a pair of interfering, actinic beams onto the medium, and during the impinging step, advancing the illuminated portion of the interference pattern relative to the medium. The advancement is carded out without changing the phase, or registration, of the interference pattern. According to one embodiment of the invention, a grating having a spatially dependent period is produced by varying the wavelength or the intersection angle of the actinic beams during the advancement. According to a second embodiment of the invention, a grating having a spatially dependent refractive index perturbation is produced by varying the dose of actinic radiation received by the medium during the advancement.

FIELD OF THE INVENTION

This invention pertains to the processing of photosensitive materials toform optical elements, and more specifically to the formation of passiveoptical components that are integrated with waveguiding articles such asoptical fibers.

ART BACKGROUND

Along with photoresists and the like, certain optical media, includingat least some silica-based optical fibers, can be modified by exposureto electromagnetic radiation in an appropriate spectral range. (Suchradiation, typically ultraviolet radiation, is referred to below as"actinic" radiation.) That is, exposure of a photosensitive opticalfiber (or other optical medium) to actinic radiation may cause therefractive index to change in the exposed portion of the medium. Aperiodic pattern can be imposed on the impinging radiation by, e.g.,superimposing a pair of beams of substantially monochromatic radiationfrom, e.g., a laser, to create an interference pattern. If two beams ofwavelength λ, intersect at an intersection angle φ, the resultinginterference pattern will have a period Λ given by ##EQU1## When such apatterned radiation field impinges on a optical fiber or other opticalwaveguide having a core of the appropriate photosensitivity, acorresponding pattern is imposed on the core in the form of periodic (orquasiperiodic) fluctuations in the core refractive index. Such apattern, which is often referred to as a "Bragg grating" or a"distributed Bragg reflector (DBR)" can behave as a spectrally selectivereflector for electromagnetic radiation. Bragg gratings formed in thismanner are particularly useful as end-reflectors in optical fiberlasers. These Bragg gratings are useful both because they are spectrallyselective, and because they are readily incorporated in the same opticalfiber that supports the active laser medium.

A technique for creating these Bragg gratings is described in U.S. Pat.No. 4,725,110, issued to W.H. Glenn, et al. on Feb. 16, 1988, and U.S.Pat. No. 4,807,950, issued to W.H. Glenn, et al. on Feb. 28, 1989. Anoptical fiber laser having a DBR-terminated cavity is described in G.A.Ball and W.W. Morey, "Continuously tunable single-mode erbium fiberlaser", Optics Left. 17 (1992) 420-422.

Bragg gratings are useful as passive optical components for otherapplications besides end-reflectors in fiber lasers. For example, Bragggratings are useful as spectral filters for wavelength-divisionmultiplexing and other optical signal-processing applications. Anoptical filter which comprises a Bragg grating formed in an opticalfiber is described in U.S. Pat. No. 5,007,705, issued to W.W. Morey, etal. on Apr. 16, 1991.

Similar techniques are useful for forming a grating pattern in aphotosensitive medium such as a photoresist overlying a substrate. Thesubstrate is lithographically processed after exposure and developmentof the resist.

For some applications, it is desirable to provide a Bragg grating thatis quasiperiodic instead of periodic. That is, the period of the grating(i.e., the linear distance, along the propagation axis, betweensuccessive peaks or valleys of the refractive index profile) is not aconstant, but instead changes in a predetermined fashion along thepropagation axis. The most common quasiperiodic grating is one in whichthe period increases or decreases as a function, typically anapproximately linear function, of position along the propagation axis.Such a grating is referred to as a "chirped" grating. Chirped gratingsare useful, inter alia, for making broadband optical reflectors. Anapplication of chirped gratings in optical fiber communication lasers isdescribed in co-pending U.S. patent application Ser. No. 07/827,249,filed by R. Adar et al. on Jan. 29, 1992. An application of chirping toremove undesirable structure from grating reflectivity spectra isdescribed in the co-pending U.S. patent application entitled "Method forForming Distributed Bragg Reflectors in Optical Media", filed by V.Mizrahi et al.

In the conventional method for making chirped gratings (inphotoresists), the interfering beams that impinge upon thephotosensitive medium are not collimated. Instead, each is made todiverge at a predetermined divergence angle. As a consequence of thedivergence of the beams, there lacks a single, well-defined angle ofintersection between the beams. Instead, there is an effective angle ofintersection that depends upon position (measured along the propagationaxis of the photosensitive medium) within the interference pattern. As aresult, a grating is formed that has a spatially dependent period. Thismethod is described in X. Mai, et al., "Simple versatile method forfabricating guided-wave gratings", Appl. Optics, 24 (1985) 3155-3161.

This conventional method is disadvantageous because it cannot be used tomake a grating in which the period has an arbitrary spatial dependence.Instead, this dependence can only take a form that is accessible by themethod of diverging the beams.

SUMMARY OF THE INVENTION

We have discovered a new method for making gratings, such as Bragggratings, having spatially dependent periods. In contrast to prior anmethods, the period can be independently specified in different portionsof the grating. As a result, a broad range of functional forms can bespecified for the spatial dependence.

In one embodiment, the invention involves a method for forming a gratingalong an axis, to be referred to as an "optical propagation axis", byexposing a photosensitive medium. The direction of this axis is referredto herein as the "axial" direction. The method includes the step ofproducing two collimated, non-collinear beams of electromagneticradiation having an actinic wavelength λ, i.e., a wavelength capable ofinducing refractive index changes in the medium. The two beams areimpinged on at least a portion of the medium at an intersection angle φ,such that a periodic interference pattern is created on the impingedportion. The method further includes the step of advancing theilluminated portion of the interference pattern relative to the mediumsuch that at least local coherence of the interference pattern ispreserved. The method further includes, during the advancing step, thestep of changing the product ##EQU2## such that the interference patternhas a spatially varying period.

In a second embodiment of the invention, a Bragg grating is formed byproducing interfering, actinic beams, impinging them on a photosensitiveoptical medium, and advancing the illuminated portion of theinterference pattern relative to the medium, as recited above, resultingin formation of a refractive index perturbation in the medium. Theinventive method, in this embodiment, further includes, during theadvancing step, varying the dose of actinic radiation received by thevarious points of the resulting refractive index perturbation. As aresult of this variation, the average amplitude of the perturbation ismade to vary, in the axial direction, according to a predeterminedpattern. The "average" amplitude in this sense is thespatially-dependent amplitude averaged over many, e.g. 10, gratingperiods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, optical diagram of an interferometer useful forpracticing the invention, in one embodiment.

FIG. 2 is a portion of the optical diagram of FIG. 1. According to theinvention in one embodiment, a curved mirror has been substituted forone of the planar mirrors of FIG. 1.

FIG. 3 is a block diagram of an illustrative system for practicing theinvention, according to one embodiment. The system includes apparatusfor controlling the dose of actinic radiation to the photosensitivemedium.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

For simplicity, the following discussion will be directed to forming aBragg grating in an optical fiber. It should be noted, however, thatgratings can be formed in other optical media either by direct exposureor by exposure of a photoresist followed by conventional lithographicprocessing. We intend to include these alternative media within thescope of the invention, and to include, in addition to Bragg gratings,other types of gratings such as reflection gratings.

We have found it advantageous to create the interference pattern using ascanning interferometer of a design in which the translation of asingle, translatable mirror can advance the position of the illuminatedportion of the interference pattern along the fiber, or otherphotosensitive medium, while preserving its registration, i.e., withoutchanging the phase of the interference pattern. The fiber or othermedium is kept stationary and the mirror is translated during exposureof, e.g., the fiber. As a consequence, a refractive index perturbationis readily created in, e.g., the fiber having a greater axial extentthan the illuminated portion itself.

A currently preferred interferometer 20 for carrying out such exposuresis depicted in FIG. 1, and is described in detail in U.S. Pat. No.4,093,338, issued to G.C. Bjorklund, et al. on Jun. 6, 1978. The opticalarrangement of this interferometer includes laser source 11,translatable mirror 13, rotatable mirror 22, and mirrors 14, 17, and 21.The interfering beams converge on photosensitive medium 18, which isexemplarily an optical fiber. The illuminated portion of theinterference pattern is shifted (without affecting its phase) along thefiber by translating mirror 13. The period of the interference patternis determined by the actinic wavelength, and by the rotational positionof rotatable mirror 22.

According to a preferred method for making the Bragg gratings, the fiberis first clamped into position to assure that the regions to be exposedare straight. The fiber is subjected to an effective exposure ofradiation, typically ultraviolet light. Various appropriate sources ofultraviolet light are available and known to those skilled in the art.

By way of illustration, we have found that an excimer-pumped, frequencydoubled, tunable dye laser emitting at about 245 nm is an appropriateexposure source. The use of such an exposure source is described inco-pending U.S. patent application Ser. No. 07/878791, filed on May 5,1992 by D.J. DiGiovanni, et al., now U.S. Pat. No. 5,237,576 which wehereby incorporate by reference. As discussed therein, this exposuresource is useful for making gratings in highly erbium-doped,silica-based optical fibers. These fibers are typically exposed to 2-mJpulses at a repetition rate of 20 pulses per second. A cylindrical lensfocuses the laser light into a band about 0.5 cm long and 100-200 μmwide. Typical exposures are about 30 seconds in duration. By thatmethod, Bragg gratings are readily formed with, e.g., a constant periodof about 0.5 μm.

As noted, the period A of the interference pattern formed by theintersecting, actinic beams is expressed by the product ##EQU3## Inorder to create a quasiperiodic grating having a desired spatialdependence, this product is varied while displacing the illuminatedportion of the interference pattern by translating mirror 13. Theproduct can be varied either by changing the wavelength λ, or bychanging the intersection angle φ. The wavelength is readily varied ifthe source of the actinic radiation is a tunable laser. By way ofillustration, excimer-pumped, frequency doubled, dye lasers are readilyavailable that emit light over a practical range extending from about235 nm to about 245 nm. At constant intersection angle, such an exposuresource allows the grating period to be varied by as much as about 4%over the length of the grating.

As noted, the preferred interferometer can be used to displace theilluminated portion of the interference pattern without changing itsphase; i.e., to displace it in such a way that its coherence ispreserved. However, if the wavelength is changed during thedisplacement, the interference pattern will be coherent only over ashort distance. Typical grating designs will call for a wavelengthvariation of only a fraction of one percent. Thus, the interferencepattern, even with wavelength variation, will typically be coherent (toa good approximation) over many tens of grating periods. Such aninterference pattern is referred to herein as "locally coherent."

One limitation on the chirping of gratings is imposed by the spot sizeof the interfering, actinic beams. If the Bragg wavelength changes toosteeply over this distance, displacement of the spot will cause newlywritten portions of the grating to add incoherently to portions writtenjust previously, leading to at least partial erasure of the grating. Asa rough guideline, this can be avoided if the variation δλ_(B) of theBragg wavelength λ_(B) over one spot size L_(spot) satisfies therelation ##EQU4## wherein Λ is the nominal grating period.

It is generally undesirable to vary the intersection angle, duringactinic exposure, by rotating mirror 22. Mechanical coupling between themirror bearings and the optical system can cause vibrations thatunacceptably degrade the stability of the interference pattern.Moreover, the rotation that produces the required wavelength shift willoften be too small to control in a practical way. However, there is apractical alternative to rotating mirror 22. That is, the intersectionangle is readily varied by substituting a curved mirror for one of theplanar mirrors of the optical system, as explained in more detail below.

Illustratively, an electromechanical actuator (the "translationactuator") is used to translate mirror 13, and a secondelectromechanical actuator (the "period-setting actuator") is used tochange the tuning of the light source. A programmable controller, suchas a microprocessor-based controller, is used to control both thetranslation actuator and the period-setting actuator. The controller isprogrammed to provide the desired functional relationship between theperiod in each portion of the grating and the axial position of thatportion.

If the intersection angle is varied by including a curved mirror in theoptical system, a chirped grating can be made in a simple way. Thespatial dependence of the grating period is determined by the shape ofthe reflective surface of one of the mirrors in the optical system. Asshown in FIG. 2, curved mirror 30 can be substituted for, e.g., mirror21 of the optical system. (The selection of mirror 21 for substitutionis not unique. The curved mirror can be substituted for any of theplanar mirrors in the optical system except for mirror 13.) If, forexample, mirror 30 has a spherical convex or concave surface, theresulting interference pattern will have a chirp that is approximatelylinear.

With reference to FIG. 2, it is apparent that a translation of mirror 13from x₀ to x₁ will turn beam 16 through an angle of 2α. The resultingchange in the intersection angle between beams 15 and 16 will cause thelocal grating period to change as well. Thus, a chirped grating isformed simply by translating mirror 13 while exposing the photosensitivemedium with a constant actinic wavelength. By way of illustration, achirped grating can be made using a convex, spherical mirror having aradius of curvature of about 50 m. Assuming a grating having a nominalBragg wavelength of 1.5 μm, we predict that a 1-cm displacement ofmirror 13 will lead to a total shift of the Bragg wavelength of about 19Å, or about 0.12%.

It should be noted that if the interfering beams are wide enoughrelative to the radius of curvature of mirror 30, a chirped grating canbe made even without a translation of mirror 13. This is done byreflecting at least one of the beams from mirror 30. (The other beam isoptionally reflected from a second curved mirror.)

The inventive method invites a further type of modification of the Bragggrating. That is, with reference to FIG. 3, the strength of grating 40(i.e., the amplitude of the refractive index perturbation) is related tothe duration and intensity of the actinic exposure. This strength can bemodulated as a function of axial position by modulating the dose ofactinic radiation. This dose is readily modulated by controlling (e.g.,by controller 42) the translational velocity of mirror 13, bycontrolling the emissive intensity of light source 11, or (if source 11is a pulsed light source) by controlling the pulse repetition rate ofthe light source. Of these three options, the last is currentlypreferred. That is, the repetition rate of, e.g., a pulsed,excimer-pumped, dye laser is readily controlled by programmable,microprocessor-based controller 44, to produce a Bragg grating having aspecified average refractive index profile. (By "average" profile ismeant the spatially dependent refractive index averaged over many,exemplarily ten, grating periods.) Modification of the averagerefractive index profile is useful, inter alia, for improving thespectral characteristics of Bragg gratings. One such application isdescribed, for example, in the previously cited, co-pending U.S. patentapplication filed by V. Mizrahi et al. under the title "Method forForming Distributed Bragg Reflectors in Optical Media".

We claim:
 1. A method for imposing a grating pattern on an article whichincludes a photosensitive medium and, associated with the medium, anaxis which defines an axial direction, the method comprising the stepsof:a) producing two collimated, non-collinear beams of electromagneticradiation having a actinic wavelength λ; b) impinging the actinic beamson at least a portion of the medium such that the beams intersect at anangle φ, resulting in the creation of an interference pattern periodicin the axial direction and having an illuminated portion that falls onthe impinged portion of the medium, leading to formation of a gratingpattern in the medium; and c) during the impinging step, axiallyadvancing the illuminated portion of the interference pattern relativeto the medium such that at least local coherence of the interferencepattern is preserved, wherein: d) the method further comprises, during(c), changing the wavelength λ, such that the resulting grating patternwill be quasiperiodic and will have a spatially varying period; andwherein the actinic beams have a spot size L_(spot), the resulting Bragggrating has a nominal grating period Λ, the resulting Bragg grating hasa Bragg wavelength λ_(B), and the axially advancing step and thewavelength changing step are carried out such that the fractional change##EQU5## of the Bragg wavelength over one spot size satisfies therelation: ##EQU6##
 2. A method for imposing a grating pattern on anarticle which includes a photosensitive medium, and associated with themedium, an axis which defines an axial direction, the method comprisingthe steps of:a) producing first and second collimated, non-collinearbeams of electromagnetic radiation having a actinic wavelength λ; b)impinging the actinic beams on at least a portion of the medium suchthat the beams intersect at an angle φ, resulting in the creation of aninterference pattern periodic in the axial direction and having anilluminated portion that falls on the impinged portion of the medium,leading to formation of a grating pattern in the medium; and c) duringthe impinging step, axially advancing the illuminated portion of theinterference pattern relative to the medium such that at least localcoherence of the interference pattern is preserved, wherein the methodfurther comprises: d) during the advancing step, impinging at least thefirst actinic beam onto a curved mirror surface, whereby an incidentportion and a reflected portion of the first actinic beam are defined,and displacing the incident portion relative to the curved mirrorsurface such that the reflected portion is rotated relative to thesecond actinic beam, thereby changing the angle φ, leading to formationof a grating pattern that is quasiperiodic and that has a spatiallyvarying period.
 3. The method of claim 2, wherein the curved mirrorsurface is a portion of a spherical surface, and the rotation of thereflected actinic beam portion results in a grating pattern having aperiod that varies approximately linearly in the axial direction.
 4. Amethod for imposing a grating pattern of refractive index perturbationson an article which includes a photosensitive medium, and, associatedwith the medium, an axis which defines an axial direction, the methodcomprising the steps of:a) producing two collimated, non-collinear beamsof electromagnetic radiation having a actinic wavelength λ; b) impingingthe actinic beams on at least a portion of the medium such that thebeams intersect at an angle φ, resulting in the creation of aninterference pattern periodic in the axial direction and having anilluminated portion that falls on the impinged portion of the medium,such that the medium is subjected to actinic exposure leading toformation of grating pattern in the medium; and c) during the impingingstep, axially advancing the illuminated portion of the interferencepattern relative to the medium such that at least local coherence of theinterference pattern is preserved, wherein: d) the method furthercomprises, during (c), changing the product ##EQU7## such that theresulting grating pattern is quasiperiodic and has a spatially varyingperiod; and the refractive index perturbations have an amplitude, theresulting grating pattern has a local grating strength related to saidamplitude and susceptible to variation by varying the amount of theactinic exposure, and the actinic beams have an average intensity; andwherein e) the method further comprises during (c), varying the actinicexposure by varying the average intensity such that the local gratingstrength is varied in the axial direction according to predeterminedpattern.
 5. A method for imposing a grating pattern of refractive indexperturbations, which have an average amplitude, on an article whichincludes a photosensitive medium and, associated with the medium, anaxis which defines an axial direction, the method comprising the stepsof:a) producing two collimated, non-collinear beams of electromagneticradiation having a actinic wavelength λ; b) impinging the actinic beamson at least a portion of the medium such that the beams intersect at anangle φ, resulting in the creation of an interference pattern periodicin the axial direction and having an illuminated portion that falls onthe impinged portion of the medium, leading to formation of a gratingpattern in the medium; and c) during the impinging step, axiallyadvancing the illuminated portion of the interference pattern relativeto the medium such that at least local coherence of the interferencepattern is preserved, wherein: d) the method further comprises, duringthe advancing step, changing the product ##EQU8## such that theresulting grating pattern is quasiperiodic and has a spatially varyingperiod; e) during step (c), the illuminated portion of the interferencepattern is advanced with a velocity υ; and f) the method furthercomprises, during (c), varying υ such that the average amplitude of theresulting refractive index perturbations varies in the axial directionaccording to a predetermined pattern.
 6. The method of claim 4, whereinthe actinic beams are produced by a pulsed radiation source, and theintensity-varying step comprises varying the pulse repetition rate ofthe radiation source.
 7. A method for forming a refractive indexperturbation, to be referred to as a "Bragg grating", in aphotosensitive optical medium having an optical propagation axis whichdefines an axial direction, the method comprising the steps of:a)producing two collimated, non-collinear beams of electromagneticradiation having a actinic wavelength λ; b) impinging the actinic beamson at least a portion of the medium such that the beams intersect at anangle φ, resulting in the creation of an interference pattern periodicin the axial direction and having an illuminated portion that falls onthe impinged portion of the medium; and c) axially advancing theilluminated portion of the interference pattern relative to the mediumsuch that at least local coherence of the interference pattern ispreserved, and a refractive index perturbation is produced in themedium, wherein each point of the perturbation receives a dose ofactinic radiation and the perturbation has an amplitude and a localstrength related to said amplitude; characterized in that the methodfurther includes: d) during the advancing step, varying the dose ofactinic radiation, such that the amplitude, and corresondingly, thelocal strength, of the resulting refractive index perturbation is variedin the axial direction according to a predetermined pattern.
 8. A methodfor forming a refractive index perturbation, to be referred to as aBragg grating, in a photosensitive optical medium having an opticalpropagation axis which defines an axial direction, the method comprisingthe steps of:a) producing two collimated, non-collinear beams ofelectromagnetic radiation having a actinic wavelength λ; b) impingingthe actinic beams on at least a portion of the medium such that thebeams intersect at an angle φ, resulting in the creation of aninterference pattern periodic in the axial direction and having anilluminated portion that falls on the impinged portion of the medium; c)axially advancing the illuminated portion of the interference patternwith a velocity ν relative to the medium such that at least localcoherence of the interference pattern is preserved, and a refractiveindex perturbation is produced in the medium, wherein each point of theperturbation receives a dose of actinic radiation, and the perturbationhas an average amplitude; and d) during (c), varying the velocity ν,resulting in a variation of the dose of actinic radiation, such that theaverage amplitude of the resulting refractive index perturbation variesin the axial direction according to a predetermined pattern.
 9. Themethod of claim 7, wherein each of the actinic beams has an averageintensity, and the dose-varying step comprises varying the averageintensities of the actinic beams.
 10. The method of claim 9, wherein theactinic beams are produced by a pulsed radiation source, and thedose-varying step comprises varying the pulse repetition rate of theradiation source.
 11. A method for imposing a grating pattern on anarticle which includes a photosensitive medium, the method comprisingthe steps of:a) producing two collimated, non-collinear beams ofelectromagnetic radiation having an actinic wavelength; and b) impingingthe actinic beams on at least a portion of the medium such that thebeams intersect at least at one angle φ, resulting in the creation of aninterference pattern on the impinged portion, leading to formation of agrating pattern in the medium; characterized in that the impinging stepcomprises: c) reflecting at least one of the actinic beams from a curvedmirror surface onto the medium, such that the intersection angle φvaries over the impinged portion, leading to formation of a gratingpattern that has a spatially varying period.